FIG. 6 is a circuit block diagram of conventional active-noise-reduction system 200. Active-noise-reduction system 200 reduces noise by carrying out adaptive control using an adaptive notch filter. Accordingly, active-noise-reduction system 200 includes reference signal generating unit 201, adaptive filter unit 202, cancelling-sound generating unit 203, error signal detection unit 206, and filter coefficient update unit 207.
Reference signal generating unit 201 outputs a reference signal having a correlation with noise generated from noise source 208. The reference signal is input into adaptive filter unit 202 from reference signal generating unit 201. Cancelling-sound generating unit 203 outputs cancelling sound 204 based on an output from adaptive filter unit 202.
Error signal detection unit 206 outputs an error signal. Note here that the error signal is generated by interference between cancelling sound 204 and noise 205 to be controlled. Filter coefficient update unit 207 determines, by calculation, a filter coefficient based on an input of the error signal from error signal detection unit 206. Then, filter coefficient update unit 207 outputs the filter coefficient determined by calculation to adaptive filter unit 202. Herein, filter coefficient update unit 207 determines, by calculation, the filter coefficient of adaptive filter unit 202 such that the error signal is minimized.
In active-noise-reduction system 200 configured as mentioned above, since the filter coefficient of adaptive filter unit 202 is updated toward the reduction of an error signal, the error signal is reduced. Then, active-noise-reduction system 200 reduces noise by repeating the processing in a specified period.
Note here that prior art literatures relating to the invention of the present application include, for example, PTL 1.